This invention relates to self-regulating heaters and more particularly to a high voltage self-regulating heater which may be applied to a refrigeration system compressor housing to maintain the lubricant therein above a predetermined temperature level.
In conventional refrigeration compressors, a refrigerant, such as one sold under the trademark "Freon" by E. I. duPont de Nemours, & Co., may, in liquid form, migrate from the condenser into the compressor lubricant. Then, when start-up of the compressor occurs, the sudden reduction in crankcase pressure may cause the refrigerant to boil, thus causing the lubricant to foam with consequent loss of lubrication to other mechanical parts of the compressor. It has been conventional to employ a crankcase heater to maintain the compressor crankcase at a temperature above that of the rest of the refrigeration system which prevents the migration of refrigerant into the crankcase lubricant.
In the prior art, fixed constant-resistance heaters were used for heating the crankcase. However, these heaters were not self-regulating and thus required the use of heat output temperature controls and the expense and low reliability associated with them. Self-regulating sump heaters such as disclosed in copending application entitled, Self-Regulating Electric Heater, Ser. No. 706,368 have proven useful in many applications.
These self-regulating heaters employ a heater made of ceramic material having a positive temperature coefficient (PTC) of resistivity. Such heaters have a relatively low resistance at usual ambient temperatures, but after initial energization by a source of electrical power will self-heat and increase their temperature and resistance. Heat will be generated and the resistance will increase rapidly above a threshold or anomaly temperature until the heat generated balances the heat dissipated at which time the temperature and resistance stabilize with the resistance many times inital value.
In certain applications particularly where a relatively high voltage is applied across the PTC heater prior art self-regulating heaters may exhibit banding phenomena of the PTC ceramic elements because of the thickness needed to maintain the electric field at an acceptable level. This banding phenomena causes a decrease in the resistivity characteristics of the heater as a whole and in an extreme case a complete breakdown of the heater material. Attempts have been made to try to overcome this problem by using a plurality of elements in series to try to divide the voltage evenly among them but this solution has caused difficulty, particularly where the resistivity characteristics of the plurality of elements are not identical which is often the case. Even if the matching of the elements in series can be accomplished, the matching is found to add to the cost of manufacturing the heater.
Accordingly it is an object of the present invention to provide an improved self-regulating heater especially useful for high voltage applications.
It is another object to provide a self-regulating heater with a low thermal resistance.
It is still another object to provide a self-regulating heater which is easy to assemble, inexpensive in construction, and reliable in operation. Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.
Briefly, the self-regulating heater of this invention preferably comprises a D-shaped thermally conductive ceramic tube with a cap at one end defining a chamber therein. The chamber preferably contains a high thermal conductivity ceramic based potting compound and a self-regulating heater member. The heating member comprises a PTC heating element with two relatively large parallel face surfaces and two smaller parallel edge surfaces. A layer of electrically conductive material is applied to the two parallel edge surfaces yielding an ohmic contact. Two thermally conductive and electrically insulative heat sink plates are positioned to be in engagement with the two face surfaces and first and second terminals are attached to the electrically conductive edge surface for supplying electrical power to the element. After partial filling of the chamber with the ceramic potting compound and inserting the heating member with terminal means, the end of the tube opposite the cap is sealed with an RTV (Room Temperature Vulcanizing) silicone rubber compound to provide a water tight device.
In accordance with this invention the design of the heater provides for flow of electrical current through the heater material in a first direction between the ohmic contact layers. On the other hand, the location of the heat sink plates on the broad flat surfaces of the heater assures that the major flow of heat from the heater is in a second direction perpendicular to the direction of the current flow. This design of the heater provides for a device which is particularly adapted for use in high voltage applications. That is, the thickness between the two heat sink plates is small so as to assure that only a minimal temperature gradient exists from the center of the element to the heat sink plates. Further this arrangement substantially eliminates the existence of temperature gradients along the direction of electrical current flow thus little or no banding occurs. On the other hand the substantial thickness of material between the ohmic contact layers on the heater assures that a substantial voltage can be applied across the heater material without breakdown of the PTC material in the heater.